Large-scale heating and cooling systems use air, water and/or steam as the heating and cooling media. Pasteurization or sterilization systems conventionally use water and steam as the heating medium and water as the cooling medium. It is well known that the conventional form and function of large-scale foodstuff heating and cooling systems present major issues to manufacturing, especially in relation to product consistency and finished product quality. This is particularly true in large volume continuous systems, where cooking temperatures are elevated to meet essential microbial safety regulations and product with low thermal conductivity properties is being processed. Such products and conditions usually require extended periods of cooking and holding at elevated temperatures to achieve the necessary safety requirements.
The slow temperature rise associated with such continuous, high volume systems frequently results in significant uneven, temperature distribution. However it is the protracted high temperature holding periods to ensure effective microbial treatment that leads to overcooking of product and a significant resultant deterioration in food quality. This overcooking is not primarily due to the protracted heating cycle but is, more frequently, a consequence of a slow cooling of the product that follows the completion of the cooking and holding cycles. Pasteurization temperatures are well above optimal temperatures needed for cooking most foodstuffs, but those needed for sterilization are substantially higher still as the medium used to achieve these elevated final temperatures is primarily pressurized steam which has both a considerable latent heat content as well as a very poor thermal conductivity. Consequently, it needs very considerable amounts of energy and thermal transfer to effectively start the cooling cycle. It is the inherent inefficiencies in this part of the processing cycle that make the greatest contribution to the general over-processing and resultant deterioration in finished product quality.
Dry and wet cooking and cooling systems present different issues. The rate of thermal conductivity in air is around 20 times less efficient than that of water. While cooking times take considerably longer in air when compared with wet cooking systems, the much lower thermal conductivity makes holding times and holding temperatures much easier to maintain and control.
Similarly, cooling systems using air as the cooling medium are much slower at equivalent temperatures compared with liquid cooling systems. But much larger volumes of substantially cooler air at accelerated velocities can be moved across the product surface in unit time, so improving their efficiency, overall performance and cost effectiveness.
Generally, large-scale heating and cooling systems have tended to use water and steam to provide the cooking and cooling capabilities because of the relative low cost of the medium, its relative abundance and availability and its relative safety. Most importantly, water has one of the highest rates of thermal conductivity for liquids. Unfortunately, it is this rapid thermal conductivity that causes the greatest problems.
Firstly, such high heat capacities mean that it frequently has more energy available than the receiving product can dissipate. The result is a very rapid heating of the surfaces closest to the energy source, a slower transfer of energy within the container and considerable overcooking of some parts of the container contents, particularly that closest to energy/container interfaces. To counteract such properties, heating curves are less steep than could be optimally achieved.
Conversely, cooling curves using water as the coolant medium have to be steeper, particularly at the onset of cooling, because of the considerable amounts of latent heat encountered in the steam component, especially in sterilization systems, to a lesser extent in pasteurization systems, but particularly with retort-based processing and continuous sterilization systems.
But the heating and cooling inefficiencies of water and steam-based systems are not the only major problems for these media. Both are extremely corrosive, especially at elevated temperatures, not just to the structure and function of the processing equipment but also the containers themselves. In an effort to reduce the corrosive properties of these media, considerably additional costs are incurred, both in the capital construction costs, due to the requirement to use more resistant materials within the system structure and in their maintenance and running costs where very expensive compounds/mixtures have to be frequently added to the water to reduce leaching rates and consequent increases in its corrosiveness.
Over time there is a steady build-up of dissolved solids in the cooling water, this causes a steady increase in its Redox potential and thus its corrosive properties as well as making it more difficult to buffer. This, in turn, leads to a steady increase in the amount of anti-corrosion compound needed to maintain water quality. Eventually the only practical answer is to replace the cooling medium.
All of this generates further issues. These corrosion-resisting compounds are often toxic and, as they usually, directly or indirectly, contact foodstuffs and/or foodstuff containers, their maximum concentrations are strictly controlled, often well below their optimal protective capability. Because of their toxicity, if they directly contact foodstuffs the product has to be discarded. The thermal conductivity rate of water and its corrosive nature also seriously limits the type of containers that can be used and processed within such systems and the rate of cooling that can be achieved without damaging the container.
And finally, the change of state of water from liquid to gas on heating and reversion back to liquid on cooling, necessitates considerable system structural strengthening to counter the large pressure differences encountered.
There is one final issue that makes water a far from ideal cooling medium and that is the change of state that occurs as it cools further, i.e. from liquid to solid, and the amount of energy required (and its associated cost) to achieve this change of state. While the latent energy generated in changing from a liquid to a solid is extremely useful as a ‘reservoir of cool energy’, the change of state makes the formation of any solid materials, such as ice, in any circulatory cooling system extremely dangerous and potentially damaging to expensive machinery. The falling temperature may also result in a reduced solubility of dissolved solids.
Based on the foregoing, it is obvious that replacing water as a coolant, reducing or eliminating coolant corrosive properties, improving coolant refrigerant capabilities and improving cost efficiency of coolant heat transfer are all essential to achieving any significant improvement in the overall performance of a cooling system, particularly product pasteurizing and/or sterilizing systems.
While there are many liquids or mixtures that could be used to achieve one or more of these properties, extensive regulatory requirements severely limits which compounds can be used, particularly for food application uses.
Glycerin had historically been used as an automobile anti-freeze but was later replaced by more efficient and cheaper glycol mixtures. More recently, with the main source of glycerin/glycerol coming from the manufacture of biodiesel from renewable resources and from sources other than petroleum resulting in a much lower supply cost than previously, there has been renewed interest in the use of glycerin/water and glycerin/alcohol/water mixtures as refrigerants (e.g. USPA 20080315152). Glycerin has GRAS status and although generally considered safe, it does produce a poisonous gas, Arcolein, when decomposed by excessive heat.
The raw glycerin is a byproduct of biofuel manufacturing, has a typical concentration of 60-82% glycerin but also contains numerous contaminants and other byproducts including glycols, alcohols, particularly methanol and ethanol, various organic and inorganic compounds, fatty acids and water.
Many refrigerant systems use various glycol mixtures or glycol/alcohol mixtures (c.f. U.S. Pat. No. 5,141,662 and USPA 20080048147). Unfortunately such compounds (particularly glycols and methanol) are not allowed to directly contact foodstuffs, although potentially they could be used as an indirect cooling medium. As such they have found considerable use as coolants in engine cooling systems as well as components of winter windscreen washer and antifreeze mixtures because, unlike water they remain in a liquid state when external or operating temperatures fall below 0° C. They have also been suggested as a suitable refrigerant in solar powered refrigerators (U.S. Pat. No. 7,543,455).
However, all these refrigerant/coolant media still contain significant amounts of water (30%-50% v/v or w/v). As a consequence, they will still generate considerable corrosion to metal components, particularly the system's physical structures. As such they need the addition of anti-corrosion compounds to allow them to function as coolants and antifreezes. Eventually, as the temperature falls, some of the water component will form solid ice.
We have found methods and apparatus that will allow a suitable glycerin only or glycerin/water mixture to function as an effective and efficient coolant, be composed only of GRAS approved components and without the need for anti-corrosion additives, they avoid any constraints in their application or use.